Recommended Roofers?

I am looking for a roofer to do yearly maintenance on a small flat roof/terrace. It is a new condition of my buildings insurance that I have a yearly inspection.

I have been through the National Federation of Roofing Contractors and contacted about 12 companies who are listed for SE1 but after a few weeks of waiting, only two have replied (one to say they are too busy and one to say they don't cover the area).

It seems a tricky industry and I have had several roofers work on the building in the past, but their companies have all come and gone. I really want to find a solid professional company that will be around for years to come.

If anyone has any good recommendations I would be very grateful. The property is actually in SE11 -- literally across the street from SE1, by the University of the Arts London in Elephant and Castle.